The selective removal of CO2 from mixtures of gases is necessary to help addressing environmental and energy related problems, respectively. For example, it is well known that the efficient capture of CO2 from industrial emissions will help to mitigate greenhouse effects. On the energy side, a considerable part of the natural gas reserves in the US (˜17%) is treated to remove high CO2 concentrations, as the pipeline quality for CO2 concentration is less than 2%. This “contaminant” reduces the energy content of natural gas, causing it to miss the pipeline quality requirement for minimum heating value (970 BTU/ft3). Amine absorption is the most widely used method today for the removal of CO2 from CH4 streams, but this technique is highly complex and therefore costly.
Adsorption processes using microporous and mesoporous (known together as nanoporous) materials are promising methods for the removal of CO2, avoiding regeneration and corrosion problems resulting from traditional solvent absorption methods. A number of nanoporous materials have already shown potential for this specific gas separation due to their small pore sizes and their narrow pore-size distribution. Many studies have tried to address the CO2 separation problem by using microporous materials, such as 5A, 13Xl and Clinoptilolite zeolites, mesoporous silica material SBA-15, and carbon molecular sieves (CMS), respectively. For instance, 5A-type zeolites have been used for air purification applications, such as in adsorption devices to remove excess carbon dioxide from breathing air in aerospace vehicles cabins. At a concentration of ca. 900 ppm CO2, the 5A-zeolite adsorbed approximately 1.5-wt % of the gas. It is important to point out that there have been also some efforts towards the development of membranes for CO2 removal. Tsapatsis and co-workers (incorporated herein by reference) developed a silicalite-1 membrane supported on porous alumina disks that displayed some selectivity towards CO2. The highly oriented membranes considerably improved the otherwise low selectivity exhibited by silicalite powders. Recently, Falconer and co-workers (incorporated herein by reference) reported the preparation of Na+-SAPO-34 membranes via in situ crystallization on α-alumina tubular and porous stainless steel supports. These membranes were used to separate CO2 from CH4 and other gases, achieving also good selectivities. Among all of these adsorbent alternatives, SAPO-34 materials hold the most promise and therefore are the focus of this invention.
Silicoaluminophosphates (SAPOs) are crystalline nanoporous (or microporous according to the IUPAC classification for pore diameters) materials formed by silicon, aluminum, phosphorous, and oxygen atoms in tetrahedral coordination. The atoms are arranged in an orderly fashion and form frameworks with a variety of geometries. SAPO-34 has the framework characteristics of natural zeolite Chabazite (CHA) as shown in FIG. 1. This molecular sieve includes 8-ring apertures (˜4 Å) that permit access to a 3-D channel and barrel-shaped cage system. Such geometry allows molecules with small kinetic diameters to easily diffuse through the crystal structure. SAPOs also have a framework with a net charge that varies depending upon how the silicon substitution into an aluminophosphate group is achieved. That is, if silicon is substituted for aluminum, phosphorous, or both, the resulting net charge will be +1, −1 or 0, respectively. Studies have shown that usually the second and third substitution mechanisms are present during the crystallization process. Therefore, the SAPOs framework requires the presence of counterbalance species, such as cations and/or anions. These ions affect the pore size and geometry, causing potential blockage of certain molecules and limiting the diffusion of some species through the structure. In addition, these extraframework species provide effective functionalization of the sorbent surface since they can induce specific sorbate-to-sorbent energy interactions. Several cation sites in Chabazite-like materials have been identified as seen on FIG. 1. The principal cation positions are in the center of a hexagonal prism (Site I), in the center of the six-ring window (Site II), in the cavity displaced from the six-ring window (Site II′), and near the center of the eight-ring window (Site III). Given the influence of cation sites in adsorption processes, the specification will occasionally refer to these positions.
The nature of extra framework cationic species in SAPO-34 considerably affects the sorbent performance, particularly the interaction with N2 and CH4 gases. For instance, isosteric heat of adsorption data showed that nitrogen molecule interaction with the sorbent increases according to the extra framework species as follows: Mg2+<Ti3+<Ce3+<Ca2+<Ag+<Na+<Sr2+.
Hence, it is necessary to find alternative methods that could achieve CO2 separation in a selective and effective way at ambient conditions.